"the semaphore timeout period has expired" help on home networking

for some reason...my home network has stopped working. I set it up myself..as i'm kind of a computer geek..but for some weird reason it has stopped workin since yesterday..heres the deal....i have two computers...one upstairs, one downstairs, DSL with LynkSys router. the router is upstairs connected with a cable to the pc upstairs....and wirelessly to the pc downstairs...i had it set up so i can access my music files which are downstairs, up here...the computer upstairs is really old...runing xp however..but no SP1 or anything...the pc downstairs is new..with XP SP etc...and it all worked fine til yesterday..when i got that error while tryin to access my music folder...it wont let me connect to the pc downstairs from here.....but when i go downstairs...i can still access the pc upstairs...ive tested this because i have REALVNC on both pcs...and i can access the one upstairs from downstairs...but not the one downstairs from upstairs.....as i used to be able to a week ago...if u need any more info..please ask....but i think ive posted all u guys need to kno...i just want to listen to my music agian :-( the computer upstairs doesnt have enough memory to hold all the files..thats why the music is downstairs...thank you in advance